For words that only have three letters, there is not any 4th letter. As an example with the word “include”, there are no far more letters Later on and thus have to be the word "insert". The word “addict” is within the BIP39 wordlist however, you might have used “addi” https://guidemysocial.com/story4304835/an-unbiased-view-of-bip39-seed-phrase-github